Metal single layer abrasive cutting tool having a contoured cutting surface

ABSTRACT

This invention relates to a method of cutting, comprising the steps of: 
     a) providing an abrasive cutting tool comprising: 
     i) a substrate surface having a plurality of teeth extending therefrom, and 
     ii) a single layer of abrasive grains chemically bonded to at least a portion of each tooth to define a plurality of cutting levels parallel to the substrate surface, the cutting levels comprising a first uppermost cutting level and a second uppermost cutting level, the grains having a predetermined concentration, size and toughness, 
     b) moving the substrate surface in an intended direction of rotation, 
     c) contacting the uppermost cutting level of at least one tooth to a workpiece at a point of contact, 
     d) applying a constant force to the tool directed at the point of contact, 
     wherein the constant force is sufficient to cut the workpiece, the strength of the bond is sufficient to resist peeling, the predetermined concentration, size and toughness of the plurality of grains are such that the grains of the uppermost cutting level fracture under application of the constant force, and the hardness and thickness of the teeth are such that the portion of each tooth associated with the first uppermost cutting level abrades at about the same rate as the grains of the first uppermost cutting level fracture, thereby causing essentially simultaneous removal of the grains of the first uppermost cutting level from their bond and abrasion of the portion of each tooth associated with the first uppermost cutting level, and 
     grains of the second uppermost cutting level to the worpiece.

This application is a continuation of application Ser. No. 08/616,538,filed Mar. 15, 1996, now abandoned.

BACKGROUND OF THE INVENTION

Superhard abrasive cutting tools fitted with saw blades, core bits andcutting wheels for use in construction applications are typicallyclassified as either metal matrix bonded (MB) or as metal single layer(SL). The SL cutting tool has a single layer of abrasive grain (or“grit”) bonded to a smooth metal substrate by a minimum of bond materialso that the abrasive grit is essentially exposed on the cutting surfaceof the tool. When the cutting surface is presented to the workpiece tobe cut, substantially only the abrasive grit contacts the workpiece.Since a substantial portion of the load of the tool is carried by theabrasive grit, the load on each cutting point of the grit is very high.This condition produces high penetration rates and high cutting rates.However, since the SL tool has only a single layer of abrasive, it canno longer effectively cut once that layer is dulled during operation.

The MB tool increases tool life by increasing the number of layers ofabrasive grit on the cutting surface. The typical MB tool is made byforming segments of an abrasive-containing metal matrix and attachingthose segments to the periphery of a steel substrate. Because abrasivegrit is dispersed throughout the segments, the MB cutting tool surfaceremains effective after the uppermost abrasive grains dull and areremoved. Accordingly, MB tools generally have a longer life than SLtools. However, since the abrasive grit is embedded in a metal matrix,both the metal matrix and the abrasive grit are exposed on the toolperiphery. When the MB tool is presented to the workpiece, both theabrasive grit and the metal matrix contact the workpiece, therebylowering the load on each cutting point of the grit (as compared to anSL tool) and producing lower penetration rates and lower cutting rates.

Some metal single layer tools having teeth are known in the art. In onetool, rectangular teeth are provided on the edge of a core drill, and asingle metal layer of abrasive grains are bonded to the teeth. Duringuse, the uppermost grains on the teeth cut the workpiece. However, thelarge size, high concentration and low toughness of the grains are suchthat these uppermost grains quickly dull when cutting hard surfaces suchas masonry, and the penetration rate of the tool quickly falls to zero.

In another tool, angled teeth are provided on the cutting surface of agrinding wheel, and a single metal layer of abrasive grains iselectroplated to the teeth. During use, the uppermost grains on theteeth cut the workpiece. However, the weak mechanical nature of theelectroplate bond results in the lower levels of grains being peeledfrom the tooth. Accordingly, the usefulness of the tool is limited tothe usefulness of the uppermost layer of grains.

Therefore, it is an object of the present invention to provide a cuttingtool having the high penetration rate characteristic of an SL tool andthe long life characteristic of an MB tool.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided an abrasivecutting tool comprising:

a) a substrate surface having a plurality of teeth extending therefrom,each tooth having a surface, and

b) a layer comprising abrasive grains, the layer being chemically bondedto at least a portion of the surface of each tooth to define a pluralityof cutting levels parallel to the substrate surface,

wherein the grains have a relative strength index of at least oneminute, as measured by the FEPA standard for measuring the relativestrength of saw diamonds.

Also in accordance with the present invention, there is provided amethod of cutting, comprising the steps of:

a) providing an abrasive cutting tool comprising:

i) a substrate surface having a plurality of teeth extending therefrom,each tooth having a surface, and

ii) a layer comprising abrasive grains, the layer being chemicallybonded to at least a portion of the surface of each tooth to define aplurality of cutting levels parallel to the substrate surface, thecutting levels comprising a first uppermost cutting level and a seconduppermost cutting level, the grains having a predetermined wearresistance,

b) moving the substrate surface in an intended direction of rotation,

c) contacting the uppermost cutting level of at least one tooth to aworkpiece at a point of contact,

d) applying a constant force to the tool directed at the point ofcontact,

wherein the constant force is sufficient to cut the workpiece, thestrength of the bond is sufficient to resist peeling, the predeterminedwear resistance of the grains is such that the grains of the firstuppermost cutting level fracture under application of the constantforce, and the wear resistance of the teeth are such that the portion ofthe tooth associated with the first uppermost cutting level wears atabout the same rate as the grains of the first uppermost cutting levelfracture,

thereby causing essentially simultaneous removal of the grains of thefirst uppermost cutting level from their bond and the portion of thetooth associated with the first uppermost cutting level, and

thereby exposing the grains of the second uppermost cutting level to theworkpiece.

Also in accordance with the present invention, there is provided anabrasive cutting tool comprising:

a) a substrate surface having a plurality of teeth extending therefrom,each tooth having a surface and

b) a layer comprising abrasive grains, the layer being chemically bondedto at least a portion of the surface of each tooth to define a pluralityof cutting levels parallel to the substrate surface,

wherein the substrate surface has an intended direction of movement,wherein the plurality of teeth includes successive teeth havingsuccessively lower uppermost cutting levels in the direction of theintended direction of movement, thereby producing a cutting surfacehaving a negative angle of inclination with respect to the intendeddirection of movement.

Also in accordance with the present invention, there is provided anabrasive cutting tool comprising:

a) a substrate surface having a plurality of teeth extending therefrom,each tooth having a surface, and

b) a layer comprising abrasive grains, the layer being chemically bondedto at least a portion of the surface of each tooth to define a pluralityof cutting levels parallel to the substrate surface,

wherein the substrate surface has an intended direction of movement,wherein at least a portion of each tooth has a face which is inclined ata negative angle with respect to the intended direction of movement, andat least a portion of the grains are bonded to the face having thenegative angle of inclination.

Also in accordance with the present invention, there is provided anabrasive cutting tool comprising:

a) a substrate surface having a plurality of teeth extending therefrom,the teeth having a surface and a predetermined wear resistance, and

b) a layer comprising abrasive grains, the layer being chemically bondedto at least a portion of the surface of each tooth to define a pluralityof cutting levels parallel to the substrate surface, the grains having apredetermined wear resistance,

wherein the wear resistance of the teeth and the wear resistance of thegrains are predetermined such that, when a given cutting level contactsa workpiece under an optimum load, the grains of the given cutting levelwear and fracture at about the same rate as the portion of the toothassociated with the given cutting level wears away.

DESCRIPTION OF THE FIGURES

FIG. 1 shows the flat surface of a conventional SL tool.

FIG. 2 compares the time dependence of penetration rate for aconventional SL core drill with that of one embodiment of the presentinvention.

FIG. 3 shows one embodiment of the present invention wherein the teethhave a trapezoidal shape.

FIGS. 4 and 5 show the changing shape of the FIG. 3 embodiment duringuse, from a side and top view respectively.

FIG. 6 shows one embodiment of the present invention in which the teethhave a triangular shape.

FIG. 7 shows one embodiment of the present invention in which each toothcomprises a plurality of projections extending from the base todifferent heights.

FIGS. 8 and 9 show two preferred embodiments of the teeth of the presentinvention.

FIGS. 10, 11 and 12 show embodiments of the present invention whereinthe substrate is a wheel, core bit and blade, respectively.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention, the “cutting surface” of atooth is considered to be the grains which are bonded to the tooth. Aspecific “cutting level” of a tooth of the grains which are bonded tothe tooth at the same distance from the substrate surface. Similarly, aspecific “cutting level” of a tool are all of the grains which arebonded to the tool at the same distance from the substrate surface. The“uppermost” cutting level is that which is farthest away from thesubstrate surface. The portion of a tooth which is “associated” with aspecific cutting level is that slice of the tooth which is positioned atthe same distance from the substrate surface as the specified cuttinglevel. A plurality of cutting levels having “about the same number ofgrains” are those whose number of grains are between about 80% and 120%of the mean number of grains for those levels. The “concentration” ofgrit on a tooth is determined relative to the theoretical hexagonalclose packing of spheres.

It has been found that when a) the surface of a cutting tool iscontoured with teeth which allow a plurality of cutting levels to beformed, b) the grains bonded to the tooth surface have a predeterminedwear resistance which leads to failure by fracture rather than dulling,c) the bond between the grain and teeth is strong enough to resistpeeling, and d) the tooth wear resistance is predetermined to match thegrain wear resistance so that the tooth wears away at about the samerate the grains wear and fracture, the resultant tool possessesadvantages not found in the prior art tools. In particular, the tool ofthe present invention has a longer life and a higher penetration ratethan the both the conventional flat rimmed SL tool and the SL toolshaving teeth.

The longer life of the present invention is attributed to its ability tocontinually provide fresh, sharp abrasive at the leading edge of eachtooth as the cutting proceeds. See, for example, FIG. 3. When the toolis presented to the workpiece so that the substrate surface 8 isparallel to the workpiece surface, the angled (i.e., non-parallel) face4 of the cutting surface produces a similarly angled orientation withrespect to the workpiece surface. Since this portion of the cuttingsurface is now angled towards the workpiece surface, only the firstuppermost cutting level contacts the workpiece, thereby becoming theleading edge. As the grains of the first uppermost cutting level cut theworkpiece, then fracture and fall away before they significantly dull.At the same time, the portion of the tooth associated with the firstuppermost cutting level is made of material which abrades away at aboutthe same rate as the first uppermost cutting level, thereby exposing thefresh, sharp abrasive grains 3 of the second uppermost cutting level.This process repeats itself as each successively lower cutting level ofgrains ringing the tooth becomes the leading edge, cuts, dulls a bit,falls away and, upon simultaneous abrasion of the portion of the toothassociated with that cutting level, exposes the fresh cutting levelbeneath it. Accordingly, it is believed that the present invention isthe first SL tool to provide fresh cutting levels of abrasive grainduring use.

Moreover, since only a portion of the grains on the cutting surfacecontacts the workpiece at any one time, the load per grain is muchhigher than that found in a conventional flat rimmed SL tool having thesame amount of grain, leading to even higher penetration rates.

Further advantages of the present invention are best understood bycontrasting its penetration rate profile (over time) with that of aconventional (or “continuous rim”) SL core bit. In a continuous rim SLtool, as shown in FIG. 1, the substrate surface SS is essentially smoothand the grains G bonded thereto contact the workpiece W during eachcutting period. The initial penetration rate of this tool, denoted bythe Region 1A in FIG. 2, is somewhat high because the irregularity ofthe grain shapes produce few initial contact points. Soon afterwards,those initial contact points dull and essentially all of the grains onthe tool can contact the workpiece. However, because there are so manygrains contacting the workpiece at any one time, the lower load pergrain limits the cutting rate. As the grains dull, their cuttingefficiency falls and the penetration rate falls (Region 1B). Eventually,the grains dull to the point of polishing (Region 1C), and the tool'susefulness has been spent.

In one embodiment of the SL tool of the present invention, asexemplified by the trapezoid tooth design shown in FIG. 3, only thegrains 1 at the first uppermost cutting level of the tooth 2 initiallycontact the workpiece W. Since these grains are sharp, their contactproduces a very high initial penetration rate, as denoted by Region 3 Ain FIG. 2. As cutting proceeds, these grains dull and the penetrationrate falls slightly. However, before they dull significantly, thesegrains fracture and fall away, thus exposing the portion of the toothassociated with the uppermost cutting level. When that portion of thetooth is removed by the abrasive action the workpiece, the grains of thesecond uppermost cutting level 3 on both the sloped portion 4 of thetooth and the four sides of the tooth (such as side 5) become exposed tothe workpiece. Since these grains of the second uppermost cutting levelare still sharp, they cut very efficiently. However, there are many moregrains at this cutting level and a portion of the substrate now contactsthe workpiece, thus distributing the tool's load over a wider area.Accordingly, the load per grain is somewhat lessened and the penetrationrate, as denoted by Region 3B of FIG. 2, falls slightly. As the leadingedge travels still farther down the tooth, the load per contacting grainsteadily decreases as the area of the tooth contacting the workpiece andthe number of grains in the cutting level contacting the workpiece bothincrease. Consequently, the penetration rate falls still farther (region3C). When the leading edge reaches the grains 6 bonded to the right sideof the tooth 7, both the horizontal cross-sectional area of the toothand the number of grains contacting the workpiece stop increasing.Accordingly, the load per grain stabilizes and the penetration ratereaches a steady state condition (region 3D). The steady state conditioncontinues until the leading edge reaches the bottom of the tooth.

Because the steady state cutting condition is desirable, in preferredembodiments, at least about 50% (and more preferably at least about 75%)of the cutting levels have about the same number of grains. In someembodiments, this steady state region exists in each tooth. In someembodiments, a steady state region (i.e., successive cutting levelshaving about the same number of grains) is present in at least thelowermost 50% of the tooth.

FIGS. 4 and 5 display the change in the condition of the tooth andcutting levels during use, as viewed from the side of the tooth and fromthe perspective of the workpiece, respectively, wherein the hatchedregions of FIGS. 5b-5 d represent the portion of the tooth in contactwith the workpiece. FIGS. 4a and 5 a correspond to region 3 a; FIGS. 4band 5 b correspond to region 3 b, etc.

The trapezoidal tooth embodiment is preferred for two reasons. First,the sloped face 4 of the tooth restricts initial workpiece contact tothe few grains at the first uppermost cutting level, and restrictssubsequent contact to only a minimum of grains and tooth area at lowercutting levels on the face. This condition provides a very high initialcutting rate (regions 3 a-3 c of FIG. 2) which is important for creatingan initial cut in a workpiece. Second, at the lowest cutting levels, thenormal disposition of the tooth sides 5 and 7 guarantee that the numberof grains and the area of the substrate in contact with the workpieceremains constant for the duration of the operation. This conditionproduces a long steady state condition (Region 3D of FIG. 2) of asignificant penetration rate, wherein working grains are simply replacedby fresh grains before they significantly dull. This is important inoperations where extended reliability is needed.

In preferred embodiments having trapezoidal teeth, at least theuppermost 10% of the cutting surface of each tooth comprises a facedisposed at an angle of between about 5 and 60 degrees relative to thesurface substrate. More preferably, the lowest 50% of each tooth has aconstant cross sectional area parallel to the substrate surface.

Although the trapezoidal shape of the tooth of FIG. 3 providesparticular advantages, the shape of the tooth can be any shape whichprovides for gradual presentation of fresh abrasive grain to theworkpiece as the abrasive grains which have been in contact with theworkpiece fracture and fall away before they dull. For example, thetriangular shaped tooth of FIG. 6 is also considered an embodiment ofthe present invention. In this embodiment, the cutting tool comprises asubstrate surface 16 and a series of triangular teeth 12 extending fromthe surface. In this case, only the grains at the uppermost cuttinglevel 13 (those bonded to the top of each tooth 12) contact theworkpiece W, thereby defining the leading edge. As those grains are wornaway, fresh grains 14 (those at the second uppermost cutting level)become the leading edge. In its initial stages, the penetration behaviorof this tooth would be relatively similar to that of the trapezoidaltooth of FIG. 3 (Regions 3 a-3 c of FIG. 2), but with the triangulartooth contact area expanding less rapidly than that of the trapezoid(given equal base lengths L and grain loads), leading to higher initialpenetration rates. In the later stages of the triangular tooth's life,however, the tooth area contacting the workpiece continues to grow, asdoes the number of grains contacting the workpiece. Accordingly, asteady state condition is not reached and the penetration rate falls offmuch quicker. Although all of the grains in FIG. 6 are bonded to teeth,the substrate surface 16 may also have grains brazed thereon.

In other embodiments, as in FIG. 7, each tooth comprises a series ofprojections extending step-like to different lengths from the base ofthe substrate surface. In this embodiment, the grains 22 bonded to thetop of the tallest projection 23 constitute the uppermost cutting leveland uniformly contact the workpiece 27 at the beginning of theoperation, and as these grains 22 are worn away, fresh grains 24 bondedto a slightly shorter projection 25 become the leading edge of thecutting surface. This process is repeated as grains 26 on projection 27becoming the leading edge. Accordingly, the collective grains 22, 24 and26 are considered to constitute a single cutting surface disposed at anangle to the substrate surface. Similarly, a large number of thin,closely spaced, successively shorter teeth (as shown in FIG. 7a)function essentially equivalently to those in FIG. 7.

Another feature of the teeth of FIG. 7 is that the abrasive grains areapplied only to selected faces of each tooth. In this embodiment, thegrains 22 at the top of the face 23 constitute the leading edge whichcontacts the workpiece 20. The vertical faces 28-31 of the tooth do nothave any grain bonded to them. In operation, this tool moves in the leftto right direction against a workpiece (as signified by arrow A) toproduce a negative rake.

It is anticipated that, in some applications, the grains at the leadingedge of a sharply sloping face will be susceptible to undercutting.Undercutting occurs when a sharp grain in contact with the workpiece isprematurely removed because swarf has removed the underlying bond.Therefore, in some embodiments, the tool is designed to have at leastone cutting surface 4 which slopes downward toward the substrate surface8 at an angle of between about 5 and 35 degrees, and the tool is movedacross a workpiece W in a direction of negative rake (as signified byarrow B). Under these conditions, the grains 3 of the second uppermostcutting level are sufficiently close to the grains 1 of the firstuppermost cutting level to physically protect the integrity of the bondto the grains 1 of the first uppermost cutting level from the swarf,thereby preventing the undercutting of those grains 1. The preferredangle for preventing undercutting appears to be independent of grainsize, but varies with the concentration. When the concentration of theabrasive grain is about 100%, the angle required for protection isbetween about 26 and about 32 degrees. When the concentration of theabrasive grain is about 50%, the angle required for protection isbetween about 14 and about 17 degrees. When the concentration of theabrasive grain is about 25%, the angle required for protection isbetween about 9 and about 11 degrees. When the concentration of theabrasive grain is about 12%, the angle required for protection isbetween about 5 and about 7 degrees. In other words, the angle ofprotection should be no more than about 1 degree for every threepercentage points of grain concentration.

In other embodiments, the tooth has a shape as shown in FIG. 8. Thistooth 65 comprises an angled portion 62 which rises at an angle of about45 degrees on one side, a flat top 63, and a descending portion 64 onthe back side. Moving the tooth across a workpiece in the direction ofarrow C provides the desired negative rake condition.

In other embodiments, the tooth has a contour as shown in FIG. 9. Thetooth is rounded at its junction with the substrate surface (U) and atthe upper most ends (E). The teeth also have radial grooves (G). Therounded edges provide the uppermost grains with protection fromundercutting, while the radial grooves provide space for an additionallevel of abrasive grain which will become the leading edge when theuppermost grains fracture and fall away.

Since a conventional SL wheel heats up to between about 80 C and 100 Cduring use and undergoes significant thermal expansion, expansion gaps,or “gullets”, are often designed into the perimeter of the wheel. Forexample, on a typical 12 inch diameter wheel, these gaps are about{fraction (1/16)} inches wide, about ¼ inches in height, are spacedabout 2 inches apart, typically comprise no more than 5% of the circularperimeter and sometimes have abrasive grain adhering to their bottomsand sides. However, these gaps are completely different than the spacesbetween the teeth of the present invention, as there are so many grainson the long flat of a conventional SL wheel there is effectively onlyone cutting level, and the lower load per grain at that cutting levelmerely dulls and does not fracture the grains of that cutting level.

In preferred embodiments, the cutting surface of the present inventiontypically comprises an angled face comprising at least 30% of thecutting levels, preferably between 75% and 100%. Although not required,it is also preferable that the angled face include the uppermost 10% ofthe cutting levels. It is still more preferable that the angled facecomprise at least 50% and most preferably at least 75% of the uppermostcutting levels on the tooth.

The teeth of the present invention are typically spaced so that theirbase lengths L (as in FIG. 3) constitute at least about 10% of thesubstrate surface, preferably between about 30% and 60%. When the basescomprise less than 10%, the teeth are mechanically weak and prone tobending and breakage.

Typical teeth materials include fiber reinforced plastics, steel, andother appropriate materials having a structural strength of between 20Rc and 70 Rc. However, in preferred embodiments, the grains and teethare particularly designed to wear at about the same rate. The wearresistance of any material is determined by its hardness and fracturetoughness, and increasing either factor increases the wear resistance ofthe material. In the case of the layer of the present invention, thewear resistance is generally determined by the wear resistance of thegrains. In grains such as diamond or cubic boron nitride, the hardnessof the material can not be significantly varied so the grain's fracturetoughness is typically the feature which is varied. One common measureof the fracture toughness of a grain is its relative strength index, asmeasured by the FEPA standard for measuring the relative strength of sawdiamonds. In general, grains having a relative strength index of atleast one minute are desired. If the tooth of the present invention ismade of a material other than a metal, then the hardness or fracturetoughness of the material can be varied in order to match the wearresistance rates. In the case of the teeth of the present inventionbeing made of a metal such as steel, the fracture toughness is alreadyvery high and so the hardness of the metal is varied in order to providethe matching wear rate. In one particularly preferred embodiment, thetooth is made of a metal (preferably a steel) having a hardness ofbetween 38 R and 42 Ra while the grain (preferably a diamond) has arelative strength index of at least one minute. Core drilling tools ofthe present invention having a tooth hardness of 38-42 Ra (such as mildsteel) were observed to effectively cut a broad range of buildingmaterials including cured concrete, concrete block, and dense limestoneblock.

Effective replenishment of grains via the exposure of lower cuttinglevels may also depend upon the aggressiveness of the workpiece. If theteeth are too soft or too hard, the effectiveness of the tool can easilybe significantly reduced by 50% or more. Teeth having a hardness ofabout 50 Ra were ineffective in cutting limestone, as the limestonecould not abrade the harder steel quickly enough to expose sharp newgrit at lower cutting levels. Therefore, when the workpiece is hard andnon-abrasive, the teeth preferably have a hardness of about 28 Ra.Conversely, teeth having a hardness of about 28 Ra were ineffective indrilling concrete block, as the concrete block abraded the softer teethtoo rapidly, thereby removing grit which were still sharp. Therefore,when the workpiece is soft and abrasive, the teeth preferably have ahardness of about 64 Rc. In the cases of both the too-hard teeth and thetoo-soft teeth, the tool life was less than 50% of the life of the toothhaving the hardness of between 38 Ra and 42 Ra.

Because the total area of the grain contact points determines the normalstress at the point of contact with the workpiece and hence the cuttingrate, factors such as tooth thickness, and grain size, concentration andtoughness also are important factors.

The present invention can use any abrasive grains which have the correctsize, concentration and toughness to produce failure by fracture ratherthan dulling. Typically, the abrasive has a grain size of less than 1000um, preferably between 100 um and 600 um, and is typically present in aconcentration of less than 75%. In preferred embodiments, superabrasivessuch as diamond grit and cubic boron nitride and boron suboxide areused, typically in grain sizes found on conventional SL wheels. Whendiamond is used, it typically has a grain size of between about 100 and1000 um and is present in a concentration of about 50%. When cubic boronnitride is used, it typically has grain size of between about 100 and500 un, and is present in a concentration of about 50%. In general, whenthe material to be cut is very hard (i.e. has a Knoop hardness of over700 Rc), a tough, strong abrasive having a relative strength index of atleast 1 minute (as measured by the FEPA standard for measuring therelative strength of saw diamonds) is selected. Other conventionalabrasives such as seeded sol gel alumina and silicon carbide may also bedesirable in selected applications.

In one preferred embodiment, each tooth has a trapezoidal configurationwith a top length of about 1.5 mm, a bottom length of about 6.5 mm, awidth of about 2 mm, a height of about 2.8 mm, and has an angled facehaving an angle of about −45 degrees. The diamond's FEPA sizedesignation is 501 and the relative strength index is about 1.38 minutes(3300 cycles), as measured by the FEPA standard for measuring therelative strength of saw diamonds. The diamond grit concentration isabout 50%, or about 0.03 g/cm 2, and is bonded by a bronze titanium bondto teeth made of a steel having a hardness of about 38-42 Ra.

It is believed some brazes used in conventional SL tools can be used inaccordance with the present invention. Typical brazes include nickelalloys such as Ni—Cr alloys, and reactive brazes such as bronze-titaniumbrazes. The braze must provide a chemical bond which is strong enough toresist the simultaneous peeling of lower cutting levels when an uppercutting level is removed during grinding. In some instances, theabrasive grains can be coated with a metal such as titanium or tungstenfor better adherence to the braze.

The SL tool of the present invention can be advantageously used to cutany workpieces having a Knoop hardness of at least 500 and which havebeen conventionally cut by diamond-laden tools, in particular, inorganicmaterials such as glass, tile, concrete and composites having harddispersoids. It is especially desirable in masonry applications wherethe workpiece is selected from the group consisting of brick, such aslime-silica, cement block, and concrete, such as cured concrete. It maybe use d to cut ceramic refractories such as alumina, silicon carbideand silicon nitride. It may also be used in cutting softer materialslike plastics and composites.

In many hand-drilling applications involving tools of the presentinvention, the contact between the surface of the workpiece and thecutting surfaces of the tool carries a load of no more than about 30 to40 pounds.

The present invention can be used in typical SL configurations,including wheels, core bits and flat blades. Therefore, in accordancewith FIG. 10, there is provided an abrasive cutting wheel comprising:

i) a circular disc 71 having a substrate surface 72 land a plurality ofteeth 73 extending from the substrate surface, and

ii) a single layer of abrasive grains 76 chemically bonded to theperimeter of each tooth to define a cutting surface on each tooth.

When used in a cutting wheel operation, the disc is typically made ofsteel and has a diameter of between about 4 and about 40 inches; for aten inch blade, the teeth are preferably of a trapezoidal shape, totalabout 20 to 90 in number, have a height of about 0.02 to 0.20 inches, awidth of about 0.02 to 1.2 inches, and a thickness of about 0.02 to 0.12inches; the abrasive is typically diamond in the size range of 500 um,preferably between about 450 and 650 un having a relative strength indexof at least one minute, and in a concentration of about 0.01 to 0.08g/cm 2; the workpiece is typically concrete or masonry; and the teethmove along the workpiece in negative rake.

In accordance with FIG. 11, there is provided an abrasive core drillcomprising:

i) a barrel 81 comprising a plurality of teeth 83 extending therefrom,and

ii) a single layer of abrasive grains 85 chemically bonded to the teethto define a cutting surface 86 on each tooth.

When used in a core drill operation, the barrel is typically made ofsteel and has a diameter of between about 1 and about 6 inches; theprojections are preferably of trapezoid shape, total about 10 to 60 innumber, have a height of about 0.05 to 0.3 inches, a width of about 0.1to 0.5 inches and a thickness of about 0.04 to 0.12 inches; the abrasiveis typically diamond in the size range of 500 um, preferably betweenabout 430 and 540 um, having a relative strength index of at least oneminute, and in a concentration of about 0.01 to 0.08 g/cm 2; theworkpiece is typically masonry or concrete; and the teeth move along theworkpiece in negative rake.

The SL core drill of the present invention is completely different thanthe prior art steel hole saw blade discussed above having teeth coveredwith abrasive, as these hole saw blades typically use much larger, muchweaker abrasives in much greater concentrations (i.e., typically on theorder of 100%) and are used at very high applied loads to cut very softmaterials such as wood or plastics. In conventional use of this type ofhole saw, the grains on the leading edge of this tool do not fractureand fall away during grinding, but only dull. Since the grain size,toughness and grain concentration prevent grain replenishment at theleading edge, this conventional hole saw in no way suggests theadvantages found in the present invention.

In accordance with FIG. 12, there is provided an abrasive bladecomprising:

i) a blade 91 having a linear substrate surface 92 and a plurality ofteeth 93 extending from the linear substrate surface, and

ii) a single layer of abrasive grains 95 bonded to each tooth to definea cutting surface on each tooth,

When used in a blade operation, the blade is typically made of steel andhas a length of between about 3 and about 9 inches; the projections arepreferably of trapezoid shape, total about 10 to 100 in number, have aheight of about 0.05 to 0.25 inches, a width of about 0.05 to 0.12inches, and a thickness of about 0.05 to 0.15 inches; the abrasive istypically diamond in the size range of 300 to 600 um, preferably betweenabout 300 and 500 um, having a relative strength index of at least oneminute, and in a concentration of about 0.01 and 0.08 g/cm 2; theworkpiece is typically light concrete or cinder block; and the teethmove along the workpiece in a condition of negative rake.

Comparative Example I

This test examined the behavior of holes saws having flat rims. Two 2.5inch diameter hole saws having flat rims we re selected as the startingsubstrates. Bronze-titanium braze paste was applied to the rim, coveredwith about 1000 mg of diamond abrasive to produce an abrasiveconcentration of about 100%, and the braze was melted by conventionalvacuum furnacing.

One of the tools so made contained 35/40 SDA 100+ diamond abrasive.Cutting was undertaken by hand drilling at 1500 rpm with a load of 30-40pounds. This tool drilled only about 20 two-inch holes through heavyweight concrete block (“HCB”) before the diamond was dulled and thecutting rate fell below about 2 cm/min.

Comparative Example II

This test examined the behavior of hole saws having conventional MBsegments. Three MB segments containing diamond were affixed to a coredrill substantially similar to that of Comparative Example I.

These tools were tested under conditions substantially similar to thoseof Comparative Example I, and drilled an average of 400 cm through HCBat an average of 5.3 cm/min.

Comparative Example III

This test examined the behavior of a conventional hole saw havingessentially rectangular teeth to which low toughness, large sized grainwere affixed in a concentration of about 100%. These tools were testedunder conditions substantially similar to those of Comparative ExampleI, and drilled an average of only 3 cm before the cutting fell below 2cm/min.

EXAMPLE I

In this embodiment, core drills of the present invention were made using66 mm diameter outlet bits having a 2 mm wall thickness and made of 1020steel having a 38-42 Ra hardness. The contour of the cutting surface wassubstantially similar to that shown in FIG. 9.

The crown of the bit was contoured by turning a 1 mm radius onto theleading edge of the steel substrate and then milling nine 0.050 inchwide slots spaced equally around the periphery. This procedure producednine 0.402 inch wide, 0.200 high teeth. Two equally spaced radialgrooves (0.0625 inches wide, 0.0625 inches deep) were then ground intothe face of each tooth. A 0.035 inch layer of bronze titanium was thenapplied to the teeth and a layer of 35/40 SDA 100+ diamond (about 450mg) was applied to the braze surface and the resulting assembly wasplaced in a vacuum furnace for conventional braze curing.

Bits made in accordance with this example were used to hand drill heavyweight concrete HCB blocks. After about 120-140 inches of drilling, thediamond on the face of the teeth was worn away. However, the bitscontinued to cut an acceptable rate (about 5 cm/min) because of thediamond positioned in the grooves of the teeth and on the peripheralfaces of the teeth. These bits drilled another 60-80 inches of concreteafter the uppermost diamond wore away. The total depth of cut was about180-220 inches, or 490-560 cm.

EXAMPLE II

In this embodiment, tools of the present invention were made using 66 mmdiameter outlet bits having a 2 mm wall thickness and made of 1020 steelhaving a 38-42 Ra hardness. The contour of the cutting surface wassubstantially similar to that shown in FIG. 8.

A single layer of abrasive grain consisting essentially of 0.5 g of35\40 mesh SDA 100+ diamond and bronze-titanium braze was applied to thecontour of each bit.

These bits were use to drill holes in HCB blocks. Initial penetrationrates were about 3-4 inches a minute. Although the diamond on theleading edge of the contour was worn away after only a few holes, thebits continued to drill over four meters of concrete block. When thetests were suspended after each bit drilled 80 holes, the bits werestill drilling at an acceptable rate of about 1.5-2 inches (3.8-5 cm)per minute. Drilling power varied between about 350 and 550 watts overthe course of the test, while specific energy varied between about 0.30and 0.75 J/mm3. When testing resumed, the average life was found to be194 holes, or about 988 cm. The average speed was 6.7 cm/sec and theaverage power was about 414 W. Drilling was ended when then penetrationfell below 2.5 cm/min or the bit wore out. Therefore, the tool havingpointed teeth produces superior results.

EXAMPLE III

This test examined the behavior of a hole saw having a reversed toothorientation. The tooth configuration in this case was similar to that ofExample II, except that the angle of orientation was reversed so thatthe descending portion of the tooth was the leading edge, therebyproducing-positive rake. These tools were tested under conditionssubstantially similar to those of Example I. After drilling only 5 cm,the teeth were so worn that only 20% of the height of the toothremained. (In contrast, this level of wear was found in the teeth ofExample II only after a full 400 cm of drilling.) This tool drilled for396 cm before the penetration rate fell below 2.5 cm/min and had anaverage penetration rate of 5.4 cm/min. Accordingly, the tool havingteeth oriented for a negative rake produces superior results.

The results of the above tests are summarized in Table I.

TABLE I TEST Distance Drilled (cm) Speed (cm/min) C.E. I 100 — C.E. II400 5.3 C.E. III 003 — E. I 490-560 5   E. II 988 6.7 E. III 396 5.4

We claim:
 1. An abrasive cutting tool comprising: a) a monolithicsubstrate having a substrate surface with a plurality of teeth extendingtherefrom, each tooth having a contoured surface, b) a layer comprisingsuperabasive grains, the layer being chemically bonded to at least aportion of the surface of each tooth to define a plurality of cuttinglevels parallel to the substrate surface, and each cutting level on eachtooth being oriented such that a portion of each cutting level overlapsat least a portion of each other cutting level of the tooth; and c) afirst uppermost cutting level consisting of chemically bondedsuperabrasive grains and having no associated tooth portion, andsuccessive uppermost cutting levels, comprising chemically bondedsuperabrasive grains and a portion of the tooth, among the plurality ofcutting levels of each tooth; whereby after the first uppermost cuttinglevel has been worn away by cutting a workpiece, each successiveuppermost cutting level of the tooth presents to the workpiece a ring ofsuperabrasive grain around the contoured surface of the tooth, andsubstantially all superabrasive grain within the ring simultaneouslyengages in cutting.
 2. The tool of claim 1 wherein the substrate surfacehas an intended direction of movement, wherein at least a portion ofeach tooth has a face which is inclined at a negative angle with respectto the intended direction of movement, and at least a portion of thegrains are bonded to the face having the negative angle of inclination.3. The tool of claim 2 wherein the grains bonded to the face having thenegative angle of inclination are present in a concentration wherein theangle of inclination in degrees is no more than ⅓ of the grainconcentration in percent.
 4. The tool of claim 1 comprising successivecutting levels comprising at least 50% of the plurality of cuttinglevels, wherein each cutting level of the successive cutting levelscontains about the same number of grains.
 5. The tool of claim 1 whereina portion of each tooth is associated with successive cutting levelscomprising at least 50% of the cutting levels of the tooth, and whereineach cutting level of the successive cutting levels contains about thesame number of grains.
 6. The tool of claim 5 wherein the portion ofeach tooth associated with the successive cutting levels having aboutthe same number of grains has a constant cross section.
 7. The tool ofclaim 5 wherein the portion of each tooth associated with the successivecutting levels having about the same number of grains has an uppermostcross section and a lowermost cross section, and the uppermost crosssection is smaller than the lowermost cross section.
 8. The tool ofclaim 5 wherein the successive cutting levels having about the samenumber of grains comprise at least the lowermost 50% of the cuttinglevels of each tooth.
 9. The tool of claim 8 wherein the substratesurface has an intended direction of movement, wherein at least theuppermost 10% of each tooth has a face which is inclined at a negativeangle with respect to the intended direction of movement, and at least aportion of the grains are bonded to the face having the negative angleof inclination, thereby producing a trapezoidal cutting surface.
 10. Thetool of claim 1 wherein the concentration of the grain is less than 75%.11. The tool of claim 1 wherein the teeth have a hardness of betweenabout 38 and 42 Ra.
 12. A method of cutting, comprising the steps of: a)providing an abrasive cutting tool comprising: i) a substrate surfacehaving a plurality of teeth extending therefrom, each tooth having asurface, and ii) a layer comprising abrasive grains, the layer beingchemically bonded to at least a portion of the surface of each tooth todefine a plurality of cutting levels parallel to the substrate surface,the cutting levels comprising a first uppermost cutting level,consisting of chemically bonded superabrasive grains and having noassociated tooth portion, and a second uppermost cutting level,comprising chemically bonded abrasive grains and a portion of the tooth,the grains having a predetermined wear resistance, b) moving thesubstrate surface in an intended direction of rotation, c) contactingthe uppermost cutting level of at least one tooth to a workpiece at apoint of contact, d) applying a constant force to the tool directed atthe point of contact, wherein the constant force is sufficient to cutthe workpiece, the strength of the bond is sufficient to resist peeling,the predetermined wear resistance of the grains is such that the grainsof the first uppermost cutting level fracture under application of theconstant force, and the wear resistance of the teeth are such that theportion of the tooth associated with the first uppermost cutting levelwears at about the same rate as the grains of the first uppermostcutting level fracture, thereby causing essentially simultaneous removalof the grains of the first uppermost cutting level from their bond andthe portion of the tooth associated with the first uppermost cuttinglevel, and thereby exposing the grains of the second uppermost cuttinglevel to the workpiece.
 13. The method of claim 12 wherein at least aportion of each tooth has a face which is inclined at a negative anglewith respect to the intended direction of movement, and at least aportion of the grains are bonded to the face having the negative angleof inclination.
 14. The method of claim 13 wherein workpiece producesabrasive swarf when cut, and wherein the grains bonded to the facehaving the negative angle of inclination are present in a concentrationwherein the angle of inclination in degrees is no more than ⅓ of thegrain concentration in percent, thereby protecting the grains of theuppermost cutting level from undercutting.
 15. The method of claim 12comprising successive cutting levels comprising at least 50% of theplurality of cutting levels, wherein each cutting level of thesuccessive cutting levels contains about the same number of grains. 16.The method of claim 15 wherein a portion of each tooth is associatedwith successive cutting levels comprising at least 50% of the cuttinglevels of the tooth, and wherein each cutting level of the successivecutting levels contains about the same number of grains.
 17. The methodof claim 16 wherein the portion of each tooth associated with thesuccessive cutting levels having about the same number of grains has aconstant cross section.
 18. The method of claim 16 wherein the portionof each tooth associated with the successive cutting levels having aboutthe same number of grains has an uppermost cross section and a lowermostcross section, and the uppermost cross section is smaller than thelowermost cross section.
 19. The method of claim 12 wherein the graintoughness is characterized by a relative strength index of at least oneminute, as measured by the FEPA standard for measuring the relativestrength of saw diamonds.
 20. The method of claim 12 wherein the grainsize is between about 100 um and 600 um.
 21. The method of claim 16wherein the successive cutting levels having about the same number ofgrains comprise at least the lowermost 50% of the cutting levels of eachtooth.
 22. The method of claim 12 wherein the concentration of the grainis less than 75%.
 23. The method of claim 12 wherein the workpiece ismasonry having a Knoop hardness of at least 700 Rc.
 24. The method ofclaim 12 wherein the teeth have a hardness of between 38 Ra and 42 Ra.25. An abrasive cutting tool comprising: a) a monolithic substratehaving a substrate surface with a plurality of teeth extendingtherefrom, each tooth having a contoured surface, b) a layer comprisingabrasive grains, the layer being chemically bonded to at least a portionof the surface of each tooth to define a plurality of cutting levelsparallel to the substrate surface, and each cutting level on each toothbeing oriented such that a portion of each cutting level overlaps atleast a portion of each other cutting level of the tooth; and c) a firstuppermost cutting level and successive uppermost cutting levelscomprising superabrasive grains among the plurality of cutting levels ofeach tooth; wherein the substrate surface has an intended direction ofmovement, wherein at least a portion of each tooth has a face which isinclined at a negative angle with respect to the intended direction ofmovement, and at least a portion of the abrasive grains are bonded tothe face having the negative angle of inclination, and whereby after thefirst uppermost cutting level has been worn away by cutting a workpiece,each successive uppermost cutting level of the tooth presents to theworkpiece a ring of superabrasive grain around the contoured surface ofthe tooth, and substantially all superabrasive grain within the ringsimultaneously engages in cutting.
 26. The tool of claim 25 wherein atleast the uppermost 10% of each tooth comprises the face which isinclined at a negative angle with respect to the intended direction ofmovement.
 27. An abrasive cutting tool comprising: a) a substratesurface having a plurality of teeth extending therefrom, the teethhaving a surface and a predetermined wear resistance, and b) a layercomprising abrasive grains, the layer being chemically bonded to atleast a portion of the surface of each tooth to define a plurality ofcutting levels parallel to the substrate surface, the cutting levelscomprising a first uppermost cutting level consisting of chemicallybonded superabrasive grains and having no associated tooth portion, andthe grains having a predetermined wear resistance, wherein the wearresistance of the teeth and the wear resistance of the grains arepredetermined such that, when a given cutting level contacts a workpieceunder an optimum load, the grains of the given cutting level wear andfracture at about the same rate as the portion of the tooth associatedwith the given cutting level wears away.
 28. The tool of claim 27wherein the teeth have a hardness of between 38 Ra and 42 Ra.
 29. Thetool of claim 28 wherein the grains have a relative strength index of atleast one minute, as measured by the FEPA standard for determining therelative strength of saw diamonds.
 30. The abrasive cutting tool ofclaim 1, wherein the tool is selected from the group consisting of sawblades, core drills and abrasive wheels.
 31. The abrasive cutting toolof claim 25, wherein the tool is selected from the group consisting ofsaw blades, core drills and abrasive wheels.